1. Field of the Invention
The present invention relates to a wire bonding method for connecting a first bonding point and a second bonding point by a wire in a process of manufacturing a semiconductor device and more particularly to a method for forming a wire loop between the first and second bonding points.
2. Prior Art
As shown in FIG. 6, a typical wire bonding method includes a step in which a pad 2a (first bonding point) on a semiconductor chip 2 mounted on a lead frame 1 is connected to a lead 1a (second bonding point) on the lead frame 1 by a bonding wire (merely called "wire") 3; and loop shapes of the wire 3 that connects the first and second bonding points include a triangular loop shown in FIG. 6(a), and a trapezoidal loop shown in FIG. 6(b).
The triangular loop shown in FIG. 6(a) has kink 3a at the uppermost point of the neck height portion 31. The trapezoidal loop shown in FIG. 6(b) has, in addition to the kink 3a, another kink 3b in the area of the bend between the trapezoidal length portion 32 and the inclined portion 33.
Wire loop formation methods described above are disclosed, for instance, in Japanese Examined Patent Publication (Kokoku) No. H5-60657 and in Japanese Pre-Examined Patent Publication (Kokai) No. H4-318943.
Of the above two prior arts, Japanese Examined Patent Publication (Kokoku) No. H5-60657 discloses a wire bonding method in which the kink 3a is formed. This method for forming the kink 3a is described with reference to FIG. 7.
In step (a) of FIG. 7, a capillary 5 is lowered while a damper 4 which holds the wire 3 is maintained in an open state, and a ball which is formed on the tip end of the wire 3 is bonded to the first bonding point A. Then, the capillary 5 is raised slightly to point B, and the wire 3 is delivered. Next, in step (b), the capillary 5 is moved horizontally by a small amount to point C in the opposite direction from the second bonding point H (where the lead 1a is located) (see FIG. 6). Subsequently, in step (c), the capillary 5 is raised.
Generally, the operation described above in which the capillary 5 is moved in the opposite direction from the second bonding H (or point 1a) is referred to as a "reverse operation". As a result of this reverse operation, the wire 3 assumes an inclined shape between the points A and C (see step (b) of FIG. 7); and as a result a kink 3a is formed in the wire 3 by the lower end portion (tip end portion) of the capillary 5. The wire 3 that is delivered in this reverse operation from point A to point C makes up the neck height portion 31 of the loop shown in FIG. 6.
As disclosed in, for instance, Japanese Examined Patent Publication (Kokoku) No. S63-52778, back tension is applied to a part of the wire 3 that is located above the capillary 5 by blowing air onto the wire 3 from an air nozzle during the bonding process so that the wire 3 is pulled upward. However, since the air nozzle that blows out air is located above the capillary 5 in a position away from the capillary 5, and since the wire 3 is extremely slender, the strength of this back tension is inherently limited; and sufficient back tension cannot be applied to the portion of the wire 3 located near the capillary 5.
Consequently, as shown in FIG. 8, the portion of the wire 3 located near the lower end of the capillary 5 is not sufficiently inclined toward the first bonding point A, and a strong kink 3a is not formed by the lower end portion of the capillary 5. Furthermore, since the wire 3c is delivered by the reverse operation in which the capillary 5 is moved from point B to point C, the wire shape and position of the kink 3a formed by the lower end portion of the capillary 5 fluctuate. As a result of these problems, it is impossible to obtain a stable loop shape and a high shape retention force.
The kink 3a can be strengthened by increasing the amount of the reverse operation of the capillary 5 so as to eliminate the slack in the wire 3 which is caused by the delivering of the wire 3c from the lower end of the capillary 5. However, if the amount of the reverse operation is increased, an extra amount of wire (3c) is delivered by the reverse operation; and as a result, the height of the neck height portion 31 increases, and a low wire loop shape cannot be obtained.